Distributed object store for network-based content repository

ABSTRACT

A distributed object store in a network storage system uses location-independent global object identifiers (IDs) for stored data objects. The global object ID enables a data object to be seamlessly moved from one location to another without affecting clients of the storage system, i.e., “transparent migration”. The global object ID can be part of a multilevel object handle, which also can include a location ID indicating the specific location at which the data object is stored, and a policy ID identifying a set of data management policies associated with the data object. The policy ID may be associated with the data object by a client of the storage system, for example when the client creates the object, thus allowing “inline” policy management. An object location subsystem (OLS) can be used to locate an object when a client request does not contain a valid location ID for the object.

FIELD OF THE INVENTION

At least one embodiment of the present invention pertains to networkstorage systems, and more particularly, to a distributed object store ofa content repository in a network storage server system.

BACKGROUND

Network based storage, or simply “network storage”, is a common approachto backing up data, making large amounts of data accessible to multipleusers, and other purposes. In a network storage environment, a storageserver makes data available to client (host) systems by presenting orexporting to the clients one or more logical containers of data. Thereare various forms of network storage, including network attached storage(NAS) and storage area network (SAN). In a NAS context, a storage serverservices file-level requests from clients, whereas in a SAN context astorage server services block-level requests. Some storage servers arecapable of servicing both file-level requests and block-level requests.

There are several trends that are relevant to network storagetechnology. The first is that the amount of data being stored within atypical enterprise is approximately doubling from year to year. Second,there are now multiple classes of storage devices available on themarket today, each with its own performance characteristics. These twotrends together have caused users to want storage systems that mixdifferent kinds of storage in such a way that it is possible toseamlessly move data across storage tiers based on some policy orpolicies.

In addition, users often would like to apply policies to collections ofdata objects. For example, an online social networking site/servicemight want to replicate all of its original size photos (e.g., photos ofits members/users) three times, but not the thumbnail versions, sincethe thumbnails can be recreated from the originals. Yet today, settingpolicy within a storage system is a cumbersome process that has to bedone out-of-band by a system administrator. Application writers andusers cannot specify policies on groups of files/objects.

A problem associated with conventional storage systems is that the useof path names, such as in a traditional filesystem, imposes ahierarchical organization on the data, to which applications need toconform and use for different purposes, such as navigation andretrieval, access control, and data management. However, a hierarchicalorganization may not make sense for uses other than navigation andretrieval, and as a result, it can lead to inefficiencies such asduplication of content and consequent administrative overhead.

Furthermore, a hierarchical organization has also proven to beineffective for navigation and retrieval. Consider a photo that isstored under a given path name, such as “/home/eng/myname/office.jpeg”.In a traditional storage system, this name maps to a specificserver/controller, a specific volume and a specific file location (e.g.,inode number) within that volume. Thus, path names are tied to storagelocation.

SUMMARY

The techniques introduced here provide a distributed object store in anetwork storage server system. The distributed object store can be partof a content repository, which aside from the distributed object storeincludes a presentation layer, a metadata subsystem, and a policy-basedmanagement subsystem. The content repository can be implemented in amulti-node storage server cluster.

The distributed object store creates and uses system-generated,location-independent (location-transparent), global object identifiers(IDs) for sub-volume level data objects (e.g., files) managed by thestorage system. A “data object” can be any unit of data, such as a file,a block of data, or a logical unit (“LUN”). A “sub-volume level” dataobject is a data object that can be stored within a volume (definedbelow). The global object ID described herein enables the correspondingdata object to be seamlessly moved from one location to another (e.g.,from one physical or logical storage container to another) withoutaffecting clients of the storage system; this capability can be called“transparent migration”.

The global object ID can be part of a multilevel object handle, whichalso includes (in addition to the global object ID) a locationidentifier that indicates the specific location at which the data objectis stored. The multilevel object handle can also include otherinformation, such as a policy ID that identifies a set of one or moredata management policies associated with the data object. The policy IDmay be associated with the data object by a client of the storagesystem, for example at the time the client creates the data object.Embedding policy information within the object handle allows policymanagement to be implemented efficiently within the input/output (I/O)path of the server system, i.e., “inline” policy management. Forexample, in response to receiving from a client a request that includesthe object handle, the server system uses the policy ID in the objecthandle to look up in a database the particular policy or policiesassociated with that policy ID, and then applies such policy or policiesto the request and/or to the data object.

When a client submits a data access request that includes a validlocation ID (i.e., within an object handle), the server system can oftenuse that location ID to directly locate and access the target dataobject. However, in some instances the location ID in the object handlemay be invalid, such as if the target data object has been moved, or ifthe client did not provide a complete object handle. For use in suchinstances, the server system also includes an object location subsystem(OLS) to locate the target data object. The OLS includes a datastructure that maps global object IDs to corresponding valid(up-to-date) location IDs of data objects. The server system furthermaintains a namespace which is independent of the OLS mapping structureand which includes a mapping of path names to global object IDs of thedata objects stored in the server system. A “namespace”, as the term isused herein, is a mechanism for allowing end users or applications toname and organize data objects (which may, for example, providehierarchical naming and/or organization of data, such as adirectory/file structure). The namespace together with the OLS providesa layer of indirection between (i.e., provides a logical separation of)path names and storage locations of the stored data objects. Thisseparation facilitates transparent migration (i.e., an object can bemoved without affecting its name), and moreover, it enables anyparticular data object to be represented by multiple paths names,thereby facilitating navigation. In particular, this allows theimplementation of a hierarchical protocol such as NFS or CIFS on top ofan object store, while at the same time maintaining the ability to dotransparent migration.

Other aspects of the technique will be apparent from the accompanyingfigures and from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention are illustrated by wayof example and not limitation in the figures of the accompanyingdrawings, in which like references indicate similar elements.

FIG. 1 illustrates a network storage environment in which the presentinvention can be implemented.

FIG. 2 illustrates a clustered network storage environment in which thepresent invention can be implemented.

FIG. 3 is a high-level block diagram showing an example of the hardwarearchitecture of a storage controller that can implement one or morestorage server nodes.

FIG. 4 illustrates an example of a storage operating system of a storageserver node.

FIG. 5 illustrates the overall architecture of a content repositoryaccording to one embodiment.

FIG. 6 illustrates how a content repository can be implemented in theclustered architecture of FIGS. 2 through 4.

FIG. 7 illustrates a multilevel object handle.

FIG. 8 is a flow diagram showing an example of the overall process bywhich a distributed object store services a data access request from aclient.

FIG. 9 is a flow diagram showing an example of a process of in-linepolicy management.

FIG. 10 is a flow diagram showing an example of a process of associatinga data object with one or more policies.

FIG. 11 illustrates how the OLS can use a mapping structure to map aglobal object ID of an object to a corresponding location ID and policyID.

DETAILED DESCRIPTION

References in this specification to “an embodiment”, “one embodiment”,or the like, mean that the particular feature, structure orcharacteristic being described is included in at least one embodiment ofthe present invention. Occurrences of such phrases in this specificationdo not necessarily all refer to the same embodiment.

System Environment

FIGS. 1 and 2 show, at different levels of detail, a networkconfiguration in which the techniques introduced here can beimplemented. In particular, FIG. 1 shows a network data storageenvironment, which includes a plurality of client systems 104.1-104.2, astorage server system 102, and computer network 106 connecting theclient systems 104.1-104.2 and the storage server system 102. As shownin FIG. 1, the storage server system 102 includes at least one storageserver 108, a switching fabric 110, and a number of mass storage devices112, such as disks, in a mass storage subsystem 105. Alternatively, someor all of the mass storage devices 212 can be other types of storage,such as flash memory, solid-state drives (SSDs), tape storage, etc.

The storage server (or servers) 108 may be, for example, one of theFAS-xxx family of storage server products available from NetApp, Inc.The client systems 104.1-104.2 are connected to the storage server 108via the computer network 106, which can be a packet-switched network,for example, a local area network (LAN) or wide area network (WAN).Further, the storage server 108 is connected to the disks 112 via aswitching fabric 110, which can be a fiber distributed data interface(FDDI) network, for example. It is noted that, within the network datastorage environment, any other suitable numbers of storage serversand/or mass storage devices, and/or any other suitable networktechnologies, may be employed.

The storage server 108 can make some or all of the storage space on thedisk(s) 112 available to the client systems 104.1-104.2 in aconventional manner. For example, each of the disks 112 can beimplemented as an individual disk, multiple disks (e.g., a RAID group)or any other suitable mass storage device(s). The storage server 108 cancommunicate with the client systems 104.1-104.2 according to well-knownprotocols, such as the Network File System (NFS) protocol or the CommonInternet File System (CIFS) protocol, to make data stored on the disks112 available to users and/or application programs. The storage server108 can present or export data stored on the disk 112 as volumes to eachof the client systems 104.1-104.2. A “volume” is an abstraction ofphysical storage, combining one or more physical mass storage devices(e.g., disks) or parts thereof into a single logical storage object (thevolume), and which is managed as a single administrative unit, such as asingle file system. A “file system” is a structured (e.g., hierarchical)set of stored logical containers of data (e.g., volumes, logical unitnumbers (LUNs), directories, files). Note that a “file system” does nothave to include or be based on “files” per se as its units of datastorage.

Various functions and configuration settings of the storage server 108and the mass storage subsystem 105 can be controlled from a managementstation 106 coupled to the network 106. Among many other operations, adata object migration operation can be initiated from the managementstation 106.

FIG. 2 depicts a network data storage environment, which can represent amore detailed view of the environment in FIG. 1. The environment 200includes a plurality of client systems 204 (204.1-204.M), a clusteredstorage server system 202, and a computer network 206 connecting theclient systems 204 and the clustered storage server system 202. As shownin FIG. 2, the clustered storage server system 202 includes a pluralityof server nodes 208 (208.1-208.N), a cluster switching fabric 210, and aplurality of mass storage devices 212 (212.1-212.N), which can be disks,as henceforth assumed here to facilitate description. Alternatively,some or all of the mass storage devices 212 can be other types ofstorage, such as flash memory, SSDs, tape storage, etc. Note that morethan one mass storage device 212 can be associated with each node 208.

Each of the nodes 208 is configured to include several modules,including an N-module 214, a D-module 216, and an M-host 218 (each ofwhich can be implemented by using a separate software module) and aninstance of a replicated database (RDB) 220. Specifically, node 208.1includes an N-module 214.1, a D-module 216.1, and an M-host 218.1; node208.N includes an N-module 214.N, a D-module 216.N, and an M-host 218.N;and so forth. The N-modules 214.1-214.M include functionality thatenables nodes 208.1-208.N, respectively, to connect to one or more ofthe client systems 204 over the network 206, while the D-modules216.1-216.N provide access to the data stored on the disks 212.1-212.N,respectively. The M-hosts 218 provide management functions for theclustered storage server system 202. Accordingly, each of the servernodes 208 in the clustered storage server arrangement provides thefunctionality of a storage server.

The RDB 220 is a database that is replicated throughout the cluster,i.e., each node 208 includes an instance of the RDB 220. The variousinstances of the RDB 220 are updated regularly to bring them intosynchronization with each other. The RDB 220 provides cluster-widestorage of various information used by all of the nodes 208, including avolume location database (VLDB) (not shown). The VLDB is a database thatindicates the location within the cluster of each volume in the cluster(i.e., the owning D-module 216 for each volume) and is used by theN-modules 214 to identify the appropriate D-module 216 for any givenvolume to which access is requested.

The nodes 208 are interconnected by a cluster switching fabric 210,which can be embodied as a Gigabit Ethernet switch, for example. TheN-modules 214 and D-modules 216 cooperate to provide a highly-scalable,distributed storage system architecture of a clustered computingenvironment implementing exemplary embodiments of the present invention.Note that while there is shown an equal number of N-modules andD-modules in FIG. 2, there may be differing numbers of N-modules and/orD-modules in accordance with various embodiments of the techniquedescribed here. For example, there need not be a one-to-onecorrespondence between the N-modules and D-modules. As such, thedescription of a node 208 comprising one N-module and one D-moduleshould be understood to be illustrative only.

FIG. 3 is a diagram illustrating an example of a storage controller thatcan implement one or more of the storage server nodes 208. In anexemplary embodiment, the storage controller 301 includes a processorsubsystem that includes one or more processors. The storage controller301 further includes a memory 320, a network adapter 340, a clusteraccess adapter 370 and a storage adapter 380, all interconnected by aninterconnect 390. The cluster access adapter 370 includes a plurality ofports adapted to couple the node 208 to other nodes 208 of the cluster.In the illustrated embodiment, Ethernet is used as the clusteringprotocol and interconnect media, although other types of protocols andinterconnects may be utilized within the cluster architecture describedherein. In alternative embodiments where the N-modules and D-modules areimplemented on separate storage systems or computers, the cluster accessadapter 270 is utilized by the N-module 214 and/or D-module 216 forcommunicating with other N-modules and/or D-modules of the cluster.

The storage controller 301 can be embodied as a single- ormulti-processor storage system executing a storage operating system 330that preferably implements a high-level module, such as a storagemanager, to logically organize the information as a hierarchicalstructure of named directories, files and special types of files calledvirtual disks (hereinafter generally “blocks”) on the disks.Illustratively, one processor 310 can execute the functions of theN-module 214 on the node 208 while another processor 310 executes thefunctions of the D-module 216.

The memory 320 illustratively comprises storage locations that areaddressable by the processors and adapters 340, 370, 380 for storingsoftware program code and data structures associated with the presentinvention. The processor 310 and adapters may, in turn, compriseprocessing elements and/or logic circuitry configured to execute thesoftware code and manipulate the data structures. The storage operatingsystem 330, portions of which is typically resident in memory andexecuted by the processors(s) 310, functionally organizes the storagecontroller 301 by (among other things) configuring the processor(s) 310to invoke storage operations in support of the storage service providedby the node 208. It will be apparent to those skilled in the art thatother processing and memory implementations, including various computerreadable storage media, may be used for storing and executing programinstructions pertaining to the technique introduced here.

The network adapter 340 includes a plurality of ports to couple thestorage controller 301 to one or more clients 204 over point-to-pointlinks, wide area networks, virtual private networks implemented over apublic network (Internet) or a shared local area network. The networkadapter 340 thus can include the mechanical, electrical and signalingcircuitry needed to connect the storage controller 301 to the network206. Illustratively, the network 206 can be embodied as an Ethernetnetwork or a Fibre Channel (FC) network. Each client 204 can communicatewith the node 208 over the network 206 by exchanging discrete frames orpackets of data according to pre-defined protocols, such as TCP/IP.

The storage adapter 380 cooperates with the storage operating system 330to access information requested by the clients 204. The information maybe stored on any type of attached array of writable storage media, suchas magnetic disk or tape, optical disk (e.g., CD-ROM or DVD), flashmemory, solid-state disk (SSD), electronic random access memory (RAM),micro-electro mechanical and/or any other similar media adapted to storeinformation, including data and parity information. However, asillustratively described herein, the information is stored on disks 212.The storage adapter 380 includes a plurality of ports havinginput/output (I/O) interface circuitry that couples to the disks over anI/O interconnect arrangement, such as a conventional high-performance,Fibre Channel (FC) link topology.

Storage of information on disks 212 can be implemented as one or morestorage volumes that include a collection of physical storage diskscooperating to define an overall logical arrangement of volume blocknumber (VBN) space on the volume(s). The disks 212 can be organized as aRAID group. One or more RAID groups together form an aggregate. Anaggregate can contain one or more volumes/file systems.

The storage operating system 330 facilitates clients' access to datastored on the disks 212. In certain embodiments, the storage operatingsystem 330 implements a write-anywhere file system that cooperates withone or more virtualization modules to “virtualize” the storage spaceprovided by disks 212. In certain embodiments, a storage manager 460(FIG. 4) logically organizes the information as a hierarchical structureof named directories and files on the disks 212. Each “on-disk” file maybe implemented as set of disk blocks configured to store information,such as data, whereas the directory may be implemented as a speciallyformatted file in which names and links to other files and directoriesare stored. The virtualization module(s) allow the storage manager 460to further logically organize information as a hierarchical structure ofblocks on the disks that are exported as named logical unit numbers(LUNs).

In the illustrative embodiment, the storage operating system 330 is aversion of the Data ONTAP® operating system available from NetApp, Inc.and the storage manager 460 implements the Write Anywhere File Layout(WAFL®) file system. However, other storage operating systems arecapable of being enhanced or created for use in accordance with theprinciples described herein.

FIG. 4 is a diagram illustrating an example of storage operating system330 that can be used with the technique introduced here. In theillustrated embodiment the storage operating system 330 includesmultiple functional layers organized to form an integrated networkprotocol stack or, more generally, a multi-protocol engine 410 thatprovides data paths for clients to access information stored on the nodeusing block and file access protocols. The multiprotocol engine 410 incombination with underlying processing hardware also forms the N-module214. The multi-protocol engine 410 includes a network access layer 412which includes one or more network drivers that implement one or morelower-level protocols to enable the processing system to communicateover the network 206, such as Ethernet, Internet Protocol (IP),Transport Control Protocol/Internet Protocol (TCP/IP), Fibre ChannelProtocol (FCP) and/or User Datagram Protocol/Internet Protocol (UDP/IP).The multiprotocol engine 410 also includes a protocol layer whichimplements various higher-level network protocols, such as Network FileSystem (NFS), Common Internet File System (CIFS), Hypertext TransferProtocol (HTTP), Internet small computer system interface (iSCSI), etc.Further, the multiprotocol engine 410 includes a cluster fabric (CF)interface module 440 a which implements intra-cluster communication withD-modules and with other N-modules.

In addition, the storage operating system 330 includes a set of layersorganized to form a backend server 465 that provides data paths foraccessing information stored on the disks 212 of the node 208. Thebackend server 465 in combination with underlying processing hardwarealso forms the D-module 216. To that end, the backend server 465includes a storage manager module 460 that manages any number of volumes472, a RAID system module 480 and a storage driver system module 490.

The storage manager 460 primarily manages a file system (or multiplefile systems) and serves client-initiated read and write requests. TheRAID system 480 manages the storage and retrieval of information to andfrom the volumes/disks in accordance with a RAID redundancy protocol,such as RAID-4, RAID-5, or RAID-DP, while the disk driver system 490implements a disk access protocol such as SCSI protocol or FCP.

The backend server 465 also includes a CF interface module 440 b toimplement intra-cluster communication 470 with N-modules and/or otherD-modules. The CF interface modules 440 a and 440 b can cooperate toprovide a single file system image across all D-modules 216 in thecluster. Thus, any network port of an N-module 214 that receives aclient request can access any data container within the single filesystem image located on any D-module 216 of the cluster.

The CF interface modules 440 implement the CF protocol to communicatefile system commands among the modules of cluster over the clusterswitching fabric 210 (FIG. 2). Such communication can be effected by aD-module exposing a CF application programming interface (API) to whichan N-module (or another D-module) issues calls. To that end, a CFinterface module 440 can be organized as a CF encoder/decoder. The CFencoder of, e.g., CF interface 440 a on N-module 214 can encapsulate aCF message as (i) a local procedure call (LPC) when communicating a filesystem command to a D-module 216 residing on the same node or (ii) aremote procedure call (RPC) when communicating the command to a D-moduleresiding on a remote node of the cluster. In either case, the CF decoderof CF interface 440 b on D-module 216 de-encapsulates the CF message andprocesses the file system command.

In operation of a node 208, a request from a client 204 is forwarded asa packet over the network 206 and onto the node 208, where it isreceived at the network adapter 340 (FIG. 3). A network driver of layer412 processes the packet and, if appropriate, passes it on to a networkprotocol and file access layer for additional processing prior toforwarding to the storage manager 460. At that point, the storagemanager 460 generates operations to load (retrieve) the requested datafrom disk 212 if it is not resident in memory 320. If the information isnot in memory 320, the storage manager 460 indexes into a metadata fileto access an appropriate entry and retrieve a logical VBN. The storagemanager 460 then passes a message structure including the logical VBN tothe RAID system 480; the logical VBN is mapped to a disk identifier anddisk block number (DBN) and sent to an appropriate driver (e.g., SCSI)of the disk driver system 490. The disk driver accesses the DBN from thespecified disk 212 and loads the requested data block(s) in memory forprocessing by the node. Upon completion of the request, the node (andoperating system) returns a reply to the client 204 over the network206.

The data request/response “path” through the storage operating system330 as described above can be implemented in general-purposeprogrammable hardware executing the storage operating system 330 assoftware or firmware. Alternatively, it can be implemented at leastpartially in specially designed hardware. That is, in an alternateembodiment of the invention, some or all of the storage operating system330 is implemented as logic circuitry embodied within a fieldprogrammable gate array (FPGA) or an application specific integratedcircuit (ASIC), for example.

The N-module 214 and D-module 216 each can be implemented as processinghardware configured by separately-scheduled processes of storageoperating system 330; however, in an alternate embodiment, the modulesmay be implemented as processing hardware configured by code within asingle operating system process. Communication between an N-module 214and a D-module 216 is thus illustratively effected through the use ofmessage passing between the modules although, in the case of remotecommunication between an N-module and D-module of different nodes, suchmessage passing occurs over the cluster switching fabric 210. A knownmessage-passing mechanism provided by the storage operating system totransfer information between modules (processes) is the Inter ProcessCommunication (IPC) mechanism. The protocol used with the IPC mechanismis illustratively a generic file and/or block-based “agnostic” CFprotocol that comprises a collection of methods/functions constituting aCF API.

Overview of Content Repository

The techniques introduced here generally relate to a content repositoryimplemented in a network storage server system 202 such as describedabove. FIG. 5 illustrates the overall architecture of the contentrepository according to one embodiment. The major components of thecontent repository include a distributed object store 51, and objectlocation subsystem (OLS) 52, a presentation layer 53, a metadatasubsystem (MDS) 54 and a management subsystem 55. Normally there will bea single instance of each of these components in the overall contentrepository, and each of these components can be implemented in any oneserver node 208 or distributed across two or more server nodes 208. Thefunctional elements of each of these units (i.e., the OLS 52,presentation layer 53, MDS 54 and management subsystem 55) can beimplemented by specially designed circuitry, or by programmablecircuitry programmed with software and/or firmware, or a combinationthereof. The data storage elements of these units can be implementedusing any known or convenient form or forms of data storage device.

The distributed object store 51 provides the actual data storage for alldata objects in the server system 202 and includes multiple distinctsingle-node object stores 61. A “single-node” object store is an objectstore that is implemented entirely within one node. Each single-nodeobject store 61 is a logical (non-physical) container of data, such as avolume or a logical unit (LUN). Some or all of the single-node objectstores 61 that make up the distributed object store 51 can beimplemented in separate server nodes 208. Alternatively, all of thesingle-node object stores 61 that make up the distributed object store51 can be implemented in the same server node. Any given server node 208can access multiple single-node object stores 61 and can includemultiple single-node object stores 61.

The distributed object store provides location-independent addressing ofdata objects (i.e., data objects can be moved among single-node objectstores 61 without changing the data objects' addressing), with theability to span the object address space across other similar systemsspread over geographic distances. Note that the distributed object store51 has no namespace; the namespace for the server system 202 is providedby the presentation layer 53.

The presentation layer 53 provides access to the distributed objectstore 51. It is generated by at least one presentation module 48 (i.e.,it may be generated collectively by multiple presentation modules 48,one in each multiple server nodes 208). A presentation module 48 can bein the form of specially designed circuitry, or programmable circuitryprogrammed with software and/or firmware, or a combination thereof.

The presentation layer 53 essentially functions as a router, byreceiving client requests, translating them into an internal protocoland sending them to the appropriate D-module 216. The presentation layer53 provides two or more independent interlaces for accessing storeddata, e.g., a conventional NAS interface 56 and a Web Service interface60. The NAS interface 56 allows access to the object store 51 via one ormore conventional NAS protocols, such as NFS and/or CIFS. Thus, the NASinterface 56 provides a filesystem-like interface to the contentrepository.

The Web Service interface 60 allows access to data stored in the objectstore 51 via either “named object access” or “raw object access” (alsocalled “flat object access”). Named object access uses a namespace(e.g., a filesystem-like directory-tree interface for accessing dataobjects), as does NAS access; whereas raw object access usessystem-generated global object IDs to access data objects, as describedfurther below. The Web Service interface 60 allows access to the objectstore 51 via Web Service (as defined by the W3C), using for example, aprotocol such as Simple Object Access Protocol (SOAP) or a RESTful(REpresentational State Transfer-ful) protocol, over HTTP.

The presentation layer 53 further provides at least one namespace 59 foraccessing data via the NAS interface or the Web Service interface. Inone embodiment this includes a Portable Operating System Interface(POSIX) namespace. The NAS interface 56 allows access to data stored inthe object store 51 via the namespace(s) 59. The Web Service interface60 allows access to data stored in the object store 51 via either thenamespace(s) 59 (by using named object access) or without using thenamespace(s) 59 (by using “raw object access”). Thus, the Web Serviceinterface 60 allows either named object access or raw object access; andwhile named object access is accomplished using a namespace 59, rawobject access is not. Access by the presentation layer 53 to the objectstore 51 is via either a “fast path” 57 or a “slow path” 58, asdiscussed further below.

The function of the OLS 52 is to store and provide valid location IDs(and other information, such as policy IDs) of data objects, based ontheir global object IDs (these parameters are discussed further below).This is done, for example, when a client 204 requests access to a dataobject by using only the global object ID instead of a complete objecthandle including the location ID, or when the location ID within anobject handle is no longer valid (e.g., because the target data objecthas been moved). Note that the system 202 thereby provides two distinctpaths for accessing stored data, namely, a “fast path” 57 and a “slowpath” 58. The fast path 57 provides data access when a valid location IDis provided by a client 204 (e.g., within an object handle). The slowpath 58 makes use of the OLS and is used in all other instances of dataaccess. The fast path 57 is so named because a target data object can belocated directly from its (valid) location ID, whereas the slow path 58is so named because it requires a number of additional steps (relativeto the fast path) to determine the location of the target data object.

The MDS 54 is a subsystem for search and retrieval of stored dataobjects, based on metadata. It is accessed by users through thepresentation layer 53. The MDS 54 stores data object metadata, which caninclude metadata specified by users, inferred metadata and/orsystem-defined metadata. The MDS 54 also allows data objects to beidentified and retrieved by searching on any of that metadata. Themetadata may be distributed across nodes in the system. In oneembodiment where this is the case, the metadata for any particular dataobject are stored in the same node as the object itself.

As an example of user-specified metadata, users of the system can createand associate various types of tags (e.g., key/value pairs) with dataobjects, based on which such objects can be searched and located. Forexample, a user can define a tag called “location” for digital photos,where the value of the tag (e.g., a character string) indicates wherethe photo was taken. Or, digital music files can be assigned a tagcalled “mood”, the value of which indicates the mood evoked by themusic. On the other hand, the system can also generate or infer metadatabased on the data objects themselves and/or accesses to them.

There are two types of inferred metadata: 1) latent and 2)system-generated. Latent inferred metadata is metadata in a data objectwhich can be extracted automatically from the object and can be taggedon the object (examples include Genre, Album in an MP3 object, orAuthor, DocState in a Word document). System-generated inferred metadatais metadata generated by the server system 202 and includes working setinformation (e.g., access order information used for objectprefetching), and object relationship information; these metadata aregenerated by the system to enable better “searching” via metadataqueries (e.g., the system can track how many times an object has beenaccessed in the last week, month, year, and thus, allow a user to run aquery, such as “Show me all of the JPEG images I have looked at in thelast month”). System-defined metadata includes, for example, typicalfile attributes such as size, creation time, last modification time,last access time, owner, etc.

The MDS 54 includes logic to allow users to associate a tag-value pairwith an object and logic that provides two data object retrievalmechanisms. The first retrieval mechanism involves querying the metadatastore for objects matching a user-specified search criterion orcriteria, and the second involves accessing the value of a tag that wasearlier associated with a specific object. The first retrievalmechanism, called a query, can potentially return multiple objecthandles, while the second retrieval mechanism, called a lookup, dealswith a specific object handle of interest.

The management subsystem 55 includes a content management component 49and an infrastructure management component 50. The infrastructuremanagement component 50 includes logic to allow an administrative userto manage the storage infrastructure (e.g., configuration of nodes,disks, volumes, LUNs, etc.). The content management component 49 is apolicy based data management subsystem for managing the lifecycle ofdata objects (and optionally the metadata) stored in the contentrepository, based on user-specified policies or policies derived fromuser-defined SLOs. It can execute actions to enforce defined policies inresponse to system-defined trigger events and/or user-defined triggerevents (e.g., attempted creation, deletion, access or migration of anobject). Trigger events do not have to be based on user actions.

The specified policies may relate to, for example, system performance,data protection and data security. Performance related policies mayrelate to, for example, which logical container a given data objectshould be placed in, migrated from or to, when the data object should bemigrated or deleted, etc. Data protection policies may relate to, forexample, data backup and/or data deletion. Data security policies mayrelate to, for example, when and how data should be encrypted, who hasaccess to particular data, etc. The specified policies can also includepolices for power management, storage efficiency, data retention, anddeletion criteria. The policies can be specified in any known,convenient or desirable format and method. A “policy” in this context isnot necessarily an explicit specification by a user of where to storewhat data, when to move data, etc. Rather, a “policy” can be a set ofspecific rules regarding where to store what, when to migrate data,etc., derived by the system from the end user's SLOs, i.e., a moregeneral specification of the end user's expected performance, dataprotection, security, etc. For example, an administrative user mightsimply specify a range of performance that can be tolerated with respectto a particular parameter, and in response the management subsystem 55would identify the appropriate data objects that need to be migrated,where they should get migrated to, and how quickly they need to bemigrated.

The content management component 49 uses the metadata tracked by the MDS54 to determine which objects to act upon (e.g., move, delete,replicate, encrypt, compress). Such metadata may include user-specifiedmetadata and/or system-generated metadata. The content managementcomponent 49 includes logic to allow users to define policies and logicto execute/apply those policies.

FIG. 6 illustrates an example of how the content repository can beimplemented relative to the clustered architecture in FIGS. 2 through 4.Although FIG. 6 illustrates the system relative to a single server node208, it will be recognized that the configuration shown on the rightside of FIG. 6 actually can be implemented by two or more (or all) ofthe server nodes 208 in a cluster.

In one embodiment, the distributed object store 51 is implemented byproviding at least one single-node object store 61 in each of at leasttwo D-modules 216 in the system (any given D-module 216 can include zeroor more single node object stores 61). Also implemented in each of atleast two D-modules 216 in the system are: an OLS store 62 that containsmapping data structures used by the OLS 52 including valid location IDsand policy IDs; a policy store 63 (e.g., a database) that containsuser-specified policies relating to data objects (note that at leastsome policies or policy information may also be cached in the N-module214 to improve performance); and a metadata store 64 that containsmetadata used by the MDS 54, including user-specified object tags. Inpractice, the metadata store 64 may be combined with, or implemented asa part of, the single node object store 61.

The presentation layer 53 is implemented at least partially within eachN-module 214. In one embodiment, the OLS 52 is implemented partially bythe N-module 214 and partially by the corresponding M-host 218, asillustrated in FIG. 6. More specifically, in one embodiment thefunctions of the OLS 52 are implemented by a special daemon in theM-host 218 and by the presentation layer 53 in the N-module 214.

In one embodiment, the MDS 54 and management subsystem 55 are bothimplemented at least partially within each M-host 218. Nonetheless, insome embodiments, any of these subsystems may also be implemented atleast partially within other modules. For example, at least a portion ofthe content management component 49 of the management subsystem 55 canbe implemented within one or more N-modules 214 to allow, for example,caching of policies in such N-modules and/or execution/application ofpolicies by such N-module(s). In that case, the processing logic andstate information for executing/applying policies may be contained inone or more N-modules 214, while processing logic and state informationfor managing policies is stored in one or more M-hosts 218. As anotherexample, at least a portion of the MDS 54 may be implemented within oneor more D-modules 216, to allow it to access more efficiently systemgenerated metadata generated within those modules.

Administrative users can specify policies for use by the managementsubsystem 55, via a user interface provided by the M-host 218 to accessthe management subsystem 55. Further, via a user interface provided bythe M-host 218 to access the MDS 54, end users can assign metadata tagsto data objects, where such tags can be in the form of key/value pairs.Such tags and other metadata can then be searched by the MDS 54 inresponse to user-specified queries, to locate or allow specified actionsto be performed on data objects that meet user-specified criteria.Search queries received by the MDS 54 are applied by the MDS 54 to thesingle node object store 61 in the appropriate D-module(s) 216.

Distributed Object Store

As noted above, the distributed object store enables both path-basedaccess to data objects as well as direct access to data objects. Forpurposes of direct access, the distributed object store uses amultilevel object handle, as illustrated in FIG. 7. When a client 204creates a data object, it receives an object handle 71 as the responseto creating the object. This is similar to a file handle that isreturned when a file is created in a traditional storage system. Thefirst level of the object handle is a system-generated globally uniquenumber, called a global object ID, that is permanently attached to thecreated data object. The second level of the object handle is a “hint”which includes the location ID of the data object and, in theillustrated embodiment, the policy ID of the data object. Clients 204can store this object handle 71, containing the global object IDlocation ID and policy ID.

When a client 204 attempts to read or write the data object using thedirect access approach, the client includes the object handle of theobject in its read or write request to the server system 202. The serversystem 202 first attempts to use the location ID (within the objecthandle), which is intended to be a pointer to the exact location withina volume where the data object is stored. In the common case, thisoperation succeeds and the object is read/written. This sequence is the“fast path” 57 for I/O (see FIG. 5).

If, however, an object is moved from one location to another (forexample, from one volume to another), the server system 202 creates anew location ID for the object. In that case, the old location IDbecomes stale (invalid). The client may not be notified that the objecthas been moved or that the location ID is stale and may not receive thenew location ID for the object, at least until the client subsequentlyattempts to access that data object (e.g., by providing an object handlewith an invalid location ID). Or, the client may be notified but may notbe able or configured to accept or understand the notification.

The current mapping from global object ID to location ID is alwaysstored reliably in the OLS 52. If, during fast path I/O, the serversystem 202 discovers that the target data object no longer exists at thelocation pointed to by the provided location ID, this means that theobject must have been either deleted or moved. Therefore, at that pointthe server system 202 will invoke the OLS 52 to determine the new(valid) location ID for the target object. The server system 202 thenuses the new location ID to read/write the target object. At the sametime, the server system 202 invalidates the old location ID and returnsa new object handle to the client that contains the unchanged and uniqueglobal object ID, as well as the new location ID. This process enablesclients to transparently adapt to objects that move from one location toanother (for example in response to a change in policy).

An enhancement of this technique is for a client 204 never to have to beconcerned with refreshing the object handle when the location IDchanges. In this case, the server system 202 is responsible for mappingthe unchanging global object id to location ID. This can be doneefficiently by compactly storing the mapping from global object ID tolocation ID in, for example, cache memory of one or more N-modules 214.

Refer now to FIG. 8, which shows an example of the overall process bywhich the distributed object store services a data access request from aclient 204. Initially, at 801 the server system 202 receives from aclient 204 a request to access the target data object (e.g., a read orwrite request). The request at least includes a global object ID for thetarget data object. The server system 202 then determines at 802 whetherthe request includes a location ID (as noted above, in some instances aclient may provide only the global object ID with the request). If therequest includes a location ID, then the process proceeds with the fastpath I/O, i.e., to operation 803; otherwise, the process proceeds withthe slow path, to operation 808.

At 803 the distributed object store gets the location ID in the providedobject handle. Next, at 804 the server system 202 attempts to access thetarget data object according to that location ID. Part of attempting toaccess the data object is determining whether the location ID providedin the object handle is valid. In one embodiment this is accomplished byexamining a flag in metadata of the target object, where such flag isset whenever the object is deleted or moved. For example, such a flagmay exist in an inode representing the target object. If the object hasbeen deleted or moved, the location ID will be invalid.

In this regard, note that the location ID maps to an internal filehandle, which includes a number of fields. Once a D-module 216 receivesa file handle, it can determine by looking at these fields whether thefile handle is recent. The two relevant fields in this regard are thefile ID (or inode number) and the generation number. The file ID (orinode number) can be used to determine if an inode for the target dataobject exists (and so, whether the data object itself exists), and thegeneration number can be used to determine whether the file handlerefers to the correct version of the data object. The file ID (or inodenumber) maps to the data object's inode, and the generation number is acounter stored within the inode. Whenever the inode is reused (e.g., theprevious data object is deleted and a new one is created), thegeneration number within the inode is incremented. This allows aD-module 216 (and, more specifically, its storage manager 460) to detectaccess to a valid inode with an invalid generation number. Once thisoccurs the storage manager 460 in the D-module 216 returns a “Stale filehandle” error, which triggers an “Invalid Location ID” error. Thus, thefile ID can be used to determine if an inode for the target data objectexists (and so, whether the data object itself exists) and thegeneration number can be used to determine whether the file handlerefers to the correct version of the data object. If one of these is notvalid, an “Invalid Location ID” error is returned and can be used totrigger access the OLS 52 to get an updated location ID.

Referring still to FIG. 8, if the location ID in the object handle isvalid (805), then at 806 the server system 202 accesses the target dataobject according to that location ID. The server system 202 then sendsan appropriate response to the client at 807 (e.g., including therequested data in the case of a read or a confirmation in the case orwrite), and the process then ends.

If the location ID was not valid (805), then the process branches to theslow path, proceeding to operation 808. At 808 the server system 202gets the global object ID from the object handle provided by the client.At 809 the server system 202 invokes the OLS 52, passing the globalobject ID to the OLS 52. The OLS 52 then determines and returns thevalid location ID for the target data object at 810, in a manner whichis described below. The server system 202 then accesses the target dataobject at 811 according to the valid location ID, and at 812 the serversystem 202 sends an appropriate response to the client, including thenew (valid) location ID for the target object. The process then ends.

Referring again to 802, if the request from the client did not include alocation ID, the system uses the slow path, proceeding to 809 asdescribed above.

As noted above, an object handle can contain a policy ID to supportinline policy management (i.e., policy management within the normal I/Opath), which allows fast execution of policies. When a data object iscreated, the create function can also specify the policy or set ofpolicies that needs to be applied on the object. Examples of such apolicy (expressed here in natural language for simplicity) include“replicate an object twice”, “compress the object after storing it”, and“store the object on cheap, low-power disks”. One or more such policiescan be represented by a policy ID.

Each time during an object read/write or delete, the server system 202uses the policy ID encoded in the object handle to quickly look up inthe policy store the action that needs to be taken. For example, if thepolicy states “do not delete this file until 2015”, a delete operationwill fail until after that year. If for some reason a policy ID cannotbe specified (as may be the case with certain protocols, such as NFS orCIFS), a default policy or a policy based on the data object's locationor type can be applied.

FIG. 9 illustrates an example of the process of in-line policymanagement. At 901 the server system 202 receives a request from aclient 204 to access the target data object (e.g., a read or writerequest). The request includes an object handle. At 902 the serversystem 202 responds to the request by getting the policy ID containedwithin the object handle. At 903 the server system 202 determineswhether the policy ID is valid. Whether the policy ID is valid can bedetermined is essentially the same manner as done for the location ID,as described above.

If the policy ID is determined not to be valid at 903, then the processbranches to 907, where the server system 202 looks up the valid policyID for the object handle in the OLS 52, using the global object ID inthe object handle as a lookup key. The process then continues to 904.

After 907, or after the policy ID is determined to be valid at 903, theserver system 202 looks up in the policy store the policy or policiesthat correspond to the (valid) policy ID, at 904. At 905 the serversystem 202 then applies the identified policy or policies. A policy mayapply to a specific data object (e.g., “encrypt file A”). A policy canalso (or alternatively) apply to a particular client or user (e.g., “Joeis prohibited from accessing file A”) or to a particular logicalcontainer (e.g., “volume X is read-only”). The server system 202 sendsan appropriate response to the client at 906, and the process then ends.

FIG. 10 illustrates an example of the process of associating a dataobject with one or more policies. At 1001 the server system 202 receivesfrom a client 204 a request to create a data object. At 1002 the serversystem 202 checks whether the request specifies one or more policies forthe data object. If the request specifies one or more policies, then at1003 the server system 202 creates a policy ID and stores the specifiedpolicy or policies in the policy store in association with that policyID. The policies can be expressed in any known or convenient format. Theserver system 202 then creates an object handle including a globalobject ID for the object and includes the created policy ID in theobject handle at 1004. At 1005 the server system 202 creates the targetobject. The server system 202 includes in the object handle a locationID identifying the identifying the stored location of the object at1006. Finally, the server system 202 sends the client a confirmationmessage including the object handle (including the global object ID,location ID and policy ID) at 1007. The process then ends.

If it is determined at 1002 that the request from the client does notspecify a policy, then a default policy or a policy based on the dataobject's location will be used; accordingly, the process in that casebranches to 1008, where the server system 202 creates an object handlefor the object and includes the policy ID of the default (or selected)policy in the object handle. The server system 202 then proceeds to 1005and continues as described above.

Object Location Subsystem (OLS)

The OLS 52 is a mechanism the primary purpose of which is to allow avalid location ID of a data object to be determined from the object'sglobal object ID. However, the OLS 52 also allows the policy ID and/orany other metadata associated with a data object to be identified inessentially the same way. An example of how this can be implemented isdescribed now with reference to FIG. 11.

In one embodiment, each global object ID used by the server system 202is a multi-bit entity which is logically divided into multiple segments.Each segment includes at least one bit and typically includes multiplebits. In the example of FIG. 11, a global object ID 1101 is a nine-bitvalue, which is divided into three segments, X, Y and Z, each of whichincludes three bits. A first segment X represents the three mostsignificant bits of the global object ID, a second segment Y representsthe next most significant bits of the global object ID, and segment Zrepresents the three least significant bits of the global object ID.These particular numbers of bits and segments are used here only tofacilitate description; for any given system, the number of segments andbits in a global object ID can be chosen to accommodate the system'santicipated storage needs (i.e., a greater number of segments/bitsallows a greater number of data objects to be represented).

The OLS 52 includes a mapping data structure 1102 (which can be storedin the OLS store 62 in FIG. 6) that maps global object IDs to theircorresponding location IDs and policy IDs (and/or any other metadatathat may be associated with a data object). Each predefined segment ofthe global object ID is used to index into a different level of themapping data structure 1102. In the example of FIG. 11, each three-bitsegment of the global object ID can have eight possible values, e.g., 0,1, 2, . . . , 7, and therefore, can represent eight different entrieswithin a given level of the mapping data structure 1102. For example,the value of segment X is used to select the appropriate entry in thetop level of the mapping data structure 1102, the value of segment Y isused to select the appropriate entry in the next lower level of themapping data structure 1102, and the value of segment Z is used toselect the appropriate entry in the lowest level of the mapping datastructure 1102. The selected entry in the lowest level contains thecurrent (valid) location ID and policy ID of the global object ID 1101.In this way, the OLS enables the current location ID, policy ID and/orany other metadata associated with a data object to be easily locatedbased on the global object ID of the object.

In one embodiment, each node in the structure depicted in FIG. 11 is adirectory in a file system, and the traversal of the tree structure isaccomplished by a conventional directory traversal operation performedby the storage manager 460 (FIG. 4) of a D-module 216. In anotherembodiment, the leaf nodes can contain multiple mappings instead of justone. In that case, the entries in each leaf node have the form <objectid least significant bits>:<location ID, policy ID>. That is, the“remaining” least significant bits of the object ID that were not usedin the directory traversal to locate the leaf node are used as thelookup key in the directory that is the leaf node.

These nodes (both the leaves and the internal nodes) can reside on anystorage container on any D-module 216 in the system. The use of a globalnamespace in the storage cluster allows the “stitching” of these nodesinto a single tree that can be traversed using standard directory treetraversal. By spreading the tree across multiple D-modules 216, theperformance of the OLS 52 can be scaled out, and we can avoid the OLS 52becoming a centralized bottleneck.

Note also that the OLS tree can be populated “on demand” as objects arecreated that “fall” into specific areas of the tree. This approachrepresents a trade-off between space and time, i.e., the space consumedfor storing potentially unused sections of the tree versus the increasedlatency of creating objects due to having to create these OLS nodes inline during object creation.

As noted above, the server system 202 logically separates path namesfrom object handles. In a traditional storage system, a file isrepresented by a path such as “/u/foo/bar/file.doc”. In this example,“u” is a directory under the root directory “/”, “foo” is a directoryunder “u”, and so on. Each component in this path gets mapped to aspecific handle that identifies a specific storage location on aspecific storage device. Therefore, the entire path name maps to aspecific location, making it very difficult to move files around withouthaving to rename them.

The multi-level object handle technique introduced here allows theserver system 202 to break the tight relationship between path names andlocation that is characteristic of conventional storage systems. In oneembodiment, path names in the server system 202 are stored in a POSIXnamespace 59 (FIG. 5), which is maintained by the presentation layer 53and is independent of actual locations of objects. The POSIX namespace59 includes a data structure for mapping path names to correspondingglobal object IDs. By using this mapping in conjunction with the OLS 52(i.e., by mapping path name to global object ID and then mapping globalobject ID to location ID), the server system 202 can mimic a traditionalfilesystem hierarchy. In certain embodiments the global object ID isstored within the object handle presented by the NAS protocol, thusavoiding a need to lookup the mapping on every access.

The POSIX namespace 59 together with the OLS 52 thereby provides a layerof indirection between (i.e., provides a logical separation of) pathnames of stored data objects and the storage locations of the dataobjects, and also provides a layer of indirection between objectidentifiers of the stored data objects and the storage locations of thedata objects. This separation facilitates transparent migration (i.e.,an object can be moved without affecting its name), and moreover, itenables any particular data object to be represented by multiple pathsnames, thereby facilitating navigation. In particular, this allows theimplementation of hierarchical protocol such as NFS on top of an objectstore, while at the same time maintaining the ability to do transparentmigration. For example, when an object is moved to a new location, allthat is necessary is update its OLS mapping to point to the newlocation. After that, subsequent requests by path name are carried outby mapping the existing path name to the existing global object ID andthen mapping that global object ID to the new location ID.

The techniques introduced above can be implemented by programmablecircuitry programmed or configured by software and/or firmware, orentirely by special-purpose circuitry, or in a combination of suchforms. Such special-purpose circuitry (if any) can be in the form of,for example, one or more application-specific integrated circuits(ASICs), programmable logic devices (PLDs), field-programmable gatearrays (FPGAs), etc.

Software or firmware for implementing the techniques introduced here maybe stored on a machine-readable storage medium and may be executed byone or more general-purpose or special-purpose programmablemicroprocessors. A “machine-readable medium”, as the term is usedherein, includes any mechanism that can store information in a formaccessible by a machine (a machine may be, for example, a computer,network device, cellular phone, personal digital assistant (PDA),manufacturing tool, any device with one or more processors, etc.). Forexample, a machine-accessible medium includes recordable/non-recordablemedia (e.g., read-only memory (ROM); random access memory (RAM);magnetic disk storage media; optical storage media; flash memorydevices; etc.), etc.

The term “logic”, as used herein, can include, for example,special-purpose hardwired circuitry, software and/or firmware inconjunction with programmable circuitry, or a combination thereof.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be recognized that the inventionis not limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. Accordingly, the specification and drawings are to be regardedin an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. A method comprising: storing a data object in anetwork storage server system, wherein the data object is a sub-volumelevel data object; assigning an object identifier to the data object;creating an object handle in the network storage server system, theobject handle including the object identifier and a location identifier,the object identifier being independent of a location at which the dataobject is stored in the network storage server system, the locationidentifier being indicative of the location at which the data object isstored within the network storage server system, wherein the locationidentifier is distinct from the object identifier; and engaging incommunications between the network storage server system and a storageclient regarding the data object by using the object handle, wherein theobject identifier that is independent of the location at which the dataobject is stored in the network storage server system is used toidentify the data object in said communications.
 2. A method as recitedin claim 1, further comprising: maintaining, in the network storageserver system, a data structure representing a mapping between alocation-independent object identifier and a corresponding locationidentifier for each of a plurality of data objects stored by the networkstorage server system.
 3. A method as recited in claim 2, furthercomprising: migrating the data object from a first location to a secondlocation; and updating the location identifier for the data object in anentry for the data object in the data structure to correspond to thesecond location.
 4. A method as recited in claim 3, further comprising:not notifying the storage client of said migrating or said updating atleast until the storage client subsequently attempts to access the dataobject.
 5. A method as recited in claim 2, further comprising: receivingfrom the storage client, at the network storage server system, a requestrelating to the data object, the request including the object handle; inresponse to the request, determining whether a location identifierwithin the object handle in the request is valid; if the locationidentifier within the object handle in the request is valid, then usingthe location identifier within the object handle in the request toaccess the data object in response to the request; and otherwise, usingthe object identifier to look up in the data structure a valid locationidentifier for the data object, and using the valid location identifierto access the data object in response to the request.
 6. A method asrecited in claim 5, further comprising: if the location identifierwithin the object handle in the request is not valid, then sending anupdated version of the object handle from the network storage serversystem to the storage client, the updated version containing the validlocation identifier.
 7. A method as recited in claim 2, furthercomprising: receiving from the storage client, at the network storageserver system, a request relating to the data object, the requestincluding an object identifier of the data object but not including alocation identifier of the data object; using the object identifier tolook up a valid location identifier for the data object; and using thevalid location identifier to access the data object.
 8. A method asrecited in claim 1, wherein the object handle further includes a policyidentifier that identifies a set of one or more data managementpolicies.
 9. A method as recited in claim 8, further comprising:receiving from the storage client, at the network storage server system,a request relating to the data object; in response to the request, usingthe policy identifier in the object handle to look up the set of one ormore data management policies in a data structure; and applying the setof one or more data management policies in response to the request. 10.A method as recited in claim 8, wherein the set of one or more datamanagement policies has been previously associated with the data objectin response to a request from the storage client.
 11. A method asrecited in claim 10, wherein the set of one or more data managementpolicies has been previously associated with the data object in responseto a request from the storage client to create the data object.
 12. Astorage server node configured for use in a storage server cluster thatincludes a plurality of storage server nodes, the storage server nodecomprising: a processor; a D-module configured to provide backendstorage and retrieval of data objects in a nonvolatile mass storagefacility; and an N-module configured to provide network communicationservices to communicate with a storage client over a network, theN-module configured to communicate with the D-module through aninterconnect; the storage server node further configured with theability to: assign a location-independent object identifier to a dataobject, wherein the data object is a sub-volume level data object, thelocation-independent object identifier being part of an object handlefor the data object, the object handle including, in addition to thelocation-independent object identifier, a location identifier thatindicates the location at which the data object is stored in the storageserver cluster, wherein the location identifier is distinct from theobject identifier, wherein the storage server cluster and a particularstorage client use the object handle to engage in communicationsregarding the data object; and use the location-independent objectidentifier included in the object handle to identify the data object incommunications with the storage client, the location-independent objectidentifier being independent of the location at which the data object isstored in the storage server cluster.
 13. A storage server node asrecited in claim 12, the storage server node further configured tomaintain a data structure representing a mapping between alocation-independent object identifier and a corresponding locationidentifier, for each of a plurality of data objects.
 14. A storageserver node as recited in claim 13, the storage server node furtherconfigured with the ability to: migrate the data object from a firstlocation to a second location; and update the location identifier forthe data object in an entry for the data object in the data structure tocorrespond to the second location.
 15. A storage server node as recitedin claim 12, wherein the object handle further includes a policyidentifier that identifies a set of one or more data managementpolicies.
 16. A storage server node as recited in claim 15, the storageserver node further configured with the ability to: receive from thestorage client a request relating to the data object; in response to therequest, use the policy identifier in the object handle to look up theset of one or more data management policies in a data structure; andapply the set of one or more data management policies in response to therequest.
 17. A storage server node as recited in claim 16, the storageserver node further configured with the ability to associate the dataobject with the set of one or more data management policies in responseto a request from the storage client.
 18. A network storage serversystem comprising: a processor; a network interface through which tocommunicate with a plurality of storage clients over a network; astorage interface through which to communicate with a nonvolatile massstorage subsystem; and a memory storing code which, when executed by theprocessor, causes the processor to perform a plurality of operations,including storing a data object in a network storage server system,wherein the data object is a sub-volume level data object; assigning anobject identifier to the data object; creating an object handle in thenetwork storage server system, the object handle including the objectidentifier and a location identifier, the object identifier beingindependent of a location at which the data object is stored in thenetwork storage server system, the location identifier being indicativeof the location at which the data object is stored within the networkstorage server system, wherein the location identifier is distinct fromthe object identifier, wherein the network storage server system and aparticular storage client use the object handle to engage incommunications regarding the data object; and using the objectidentifier included in the object handle to identify the data object incommunications between the network storage server system and a storageclient, the object identifier being independent of the location at whichthe data object is stored in the network storage server system.
 19. Anetwork storage server system as recited in claim 18, wherein theplurality of operations further include: maintaining, in the networkstorage server system, a data structure representing a mapping between alocation-independent object identifier and a corresponding locationidentifier for each of a plurality of data objects stored by the networkstorage server system.
 20. A network storage server system as recited inclaim 19, wherein the plurality of operations further include: migratingthe data object from a first location to a second location; and updatingthe location identifier for the data object in an entry for the dataobject in the data structure to correspond to the second location.
 21. Anetwork storage server system as recited in claim 20, wherein theplurality of operations further include: not notifying the storageclient of said migrating or said updating at least until the storageclient subsequently attempts to access the data object.
 22. A networkstorage server system as recited in claim 19, wherein the plurality ofoperations further include: receiving from the storage client, at thenetwork storage server system, a request relating to the data object,the request including the object handle; in response to the request,determining whether a location identifier within the object handle inthe request is valid; if the location identifier within the objecthandle in the request is valid, then using the location identifierwithin the object handle in the request to access the data object inresponse to the request; and otherwise, using the object identifier tolook up in the data structure a valid location identifier for the dataobject, and using the valid location identifier to access the dataobject in response to the request.
 23. A network storage server systemas recited in claim 22, wherein the plurality of operations furtherinclude: if the location identifier within the object handle in therequest is not valid, then sending an updated version of the objecthandle from the network storage server system to the storage client, theupdated version containing the valid location identifier.
 24. A networkstorage server system as recited in claim 19, wherein the plurality ofoperations further include: receiving from the storage client, at thenetwork storage server system, a request relating to the data object,the request including an object identifier of the data object but notincluding a location identifier of the data object; using the objectidentifier to look up in the data structure a valid location identifierfor the data object; and using the valid location identifier to accessthe data object.
 25. A network storage server system as recited in claim18, wherein the object handle further includes a policy identifier thatidentifies a set of one or more data management policies.
 26. A networkstorage server system as recited in claim 25, wherein the plurality ofoperations further include: receiving from the storage client, at thenetwork storage server system, a request relating to the data object; inresponse to the request, using the policy identifier in the objecthandle to look up the set of one or more data management policies in adata structure; and applying the set of one or more data managementpolicies in response to the request.
 27. A network storage server systemas recited in claim 26, wherein the set of one or more data managementpolicies has been previously associated with the data object in responseto a request from the storage client to associate the set of one or moredata management policies with the data object.